Among the trends redefining 21st century biomedical diagnostics and therapeutics is the design of low-cost portable analyzers. Because light is a powerful tool in many of today's most widely used life science instruments, high intensity, low cost light engines are essential to the design and proliferation of the newest bio-analytical instruments, medical devices and miniaturized analyzers. The development of new light technology represents a critical technical hurdle in the realization of point-of-care analysis.
Lighting for life sciences is a broad and general category. Not only are the source specifications varied but so too are the equally important optical delivery requirements. Spectral and spatial lighting requirements for sensing on the head of an optical probe or within a single cell in a flowing stream differ in output power by orders of magnitude from the requirements of a multi-analyte detection scheme on an analysis chip or within the wells of a micro-titer plate. The number of colors, spectral purity, spectral and power stability, durability and switching requirements are each unique. Illuminating hundreds of thousands of spots for quantitative fluorescence within a micro-array may be best served by projection optics while microscopes set demanding specifications for light delivery to overfill the back aperture of the microscope objective within optical trains specific to each scope body and objective design.
Historically arc lamps are noted to be flexible sources in that they provide white light. The output is managed, with numerous optical elements, to select for the wavelengths of interest and for typical fluorescence based instruments, to discriminate against the emission bands. However their notorious instability and lack of durability in addition to their significant heat management requirements make them less than ideal for portable analyzers. Moreover, large power demands to drive them present a barrier to battery operation within a compact design.
Lasers require a trained user and significant safety precautions. While solid state red outputs are cost effective, the shorter wavelength outputs are typically costly, require significant maintenance and ancillary components. Color balance and drift for multi-line outputs is a serious complication to quantitative analyses based on lasers. Moreover, the bulk of fluorescence applications do not need coherent light, are complicated by speckle patterns and do not require such narrow band outputs. Overcoming each of these traits requires light management and adds cost to the implementation of lasers for most bio-analytical tools.
Finally LEDs have matured significantly within the last decades. LEDs are now available in a relatively wide range of wavelengths. However their output is significantly broad so as to require filtering. Additionally, output in the visible spectrum is profoundly reduced in the green, 500-600 nm. The LED also presents trade-offs with respect to emission wavelength dependent intensity, broad emission spectrum (spectral half width on the order of 30 nm or more), poor spectral stability, and the wide angular range of emission. In addition, the process used to manufacture LED's cannot tightly control their spectral stability; anyone wishing to use LED's in applications requiring a good spectral stability must work directly with a supplier to essentially hand-pick the LED's for the particular application. Finally, LED's generate light over a wide angular range (50% of light intensity emitted at) 70°. While optics can narrow the emission band and focus the light output, the resulting loss in power and increase in thermal output further limit the feasibility of LED light engines.
Most importantly, these light technologies cannot be readily improved for bioanalytical applications. The associated light engine market simply does not justify the large investment necessary to overcome fundamental performance limitations. As a result, analytical instrument performance and price is constrained by the light source with no clear solution in sight. Moreover the numerous manufacturers of lamps and lasers provide only a source, not an integrated light engine. Companies such as ILC Technology, Lumileds, Spectra-Physics, Sylvania and CooILED require some sort of mechanics and or electro-optics such as acousto-optic tunable filters (AOTFs), excitation filters (with a wheel or cube holder), shutters and controllers.